1. Field of the Invention
The present invention relates to field emission tubes and, more particularly, to field emission tubes for use as part of a mobile battery operated X-ray machine.
2. Related Art
X-ray machines that generate X-rays from cold field emission of electrons from the cathode of an X-ray tube are commonly employed in pulsed shadowgraph radiographs. Pulsed or flash shadowgraph radiograph was developed in 1938 as a means for observing extremely rapid motion where the subject was obscured from observation with visible light or debris. To date, flash radiography remains the principal means of observing lensed implosions and ballistic impacts over microsecond and nanosecond time scales. The majority of these X-ray systems utilize the well known Marx generator which can be viewed as a distributed transmission-storage line, consisting of n-cascaded high-voltage ceramic disc capacitors, such as barium titanate, strontium titanate or any other suitable material that has a high dielectric constant. To produce X-rays, the Marx generator is coupled to a field emission X-ray tube either directly or by coaxial cables.
In copending commonly assigned application Ser. No. 08/738,927, the present inventors disclose a mobile X-ray machine which is of the type described above and an embodiment of which is illustrated in FIG. 1. The mobile X-ray machine, which is generally denoted 10, basically comprises two aluminum enclosures 12 and 14, wherein enclosure 12 houses a field emission X-ray tube assembly 16 and a Marx generator 18, and enclosure 14 houses control electronics 20.
Considering the Marx generator 18 in more detail, a plurality of ceramic disc capacitors C1-C10 operate together with a plurality of spark-gap switches G1-G10. The capacitors C1-C10 contained in the Marx generator 18 are charged to a high voltage (H.V.) in parallel via bleeder resistors in a resistor chain (not shown). Each of the spark gap switches G1-G10 consists of two closely spaced spherical electrodes. The spark gap switches are arranged so that each charged capacitor C1-C10 in the Marx generator 18 is isolated from all other capacitors via the bleeder resistors. The spark gap switches G1-G10 are mounted along a common optical axis (not shown) together with an ultraviolet photoionization device or source 22, connected to the control electronics 20, and mounted in close proximity to the first spark gap switch G1, within the Marx generator 18. Triggering of the Marx generator 18 begins with the control electronics 20 which initiates a high voltage trigger pulse which triggers ultraviolet photoionization source 22 by way of connection path 24. In response, the U.V. photoionization source 22 emits a large flash of hard U.V. radiation. The hard U.V. radiation emitted from device 22 photoionizes spark gap G1, and the closure of this switch places the first capacitor in the Marx generator C1 in series with the second capacitor C2 in the generator 18 thereby doubling the voltage across the second spark gap switch G2. The increased voltage stress across the second spark gap together with the hard ultraviolet illumination it receives from the closure of the first spark gap switch G1 causes the second spark gap to break down quickly. This process continues at an accelerating rate until all capacitors C1-C10 in the Marx generator 18 are fully connected in series. The full Marx voltage now appears across switch G10 which is connected to a power feedthrough device 26.
Briefly considering the X-ray tube assembly 16, power feedthrough device 26 transmits the H.V. output of the Marx capacitors to the anode 28 of the X-ray tube 16, via anode tube 29. The X-ray tube assembly 16 is held within the enclosure 30 by the clamping arrangement 32. When high voltage (H.V.) pulses arrive at the anode 28 of the X-ray tube 16 these pulses establish a large potential gradient in the anode-cathode gap. This gradient produces an intense electric field at the tips of the small metal whiskers which are present on the surface of the cathode mesh 34. The whiskers (not shown) are heated by the passage of the field emission electron current and vaporize, creating a neutral plasma which acts as a virtual cathode capable of supporting a much larger current. Electrons emitted from the expanding virtual cathode are accelerated by the electric field in the anode-cathode gap and eventually collide with the anode 28 creating X-rays by the usual Bremmstrahlung and line radiation processes. Electrons continue to cross the anode-cathode gap until the X-ray tube impedance drops to a few ohms and effectively shorts the tube.
The X-ray tube or field emission tube 16 illustrated in FIG. 1 is described in more detail in the aforementioned copending application Ser. No. 08/738,927.